One issue is see with this metaphor is that having enough sand or pebbles to not have to work for a living (or chose what to work on) would be the most single m

One issue is see with this metaphor is that having enough sand or pebbles to not have to work for a living (or chose what to work on) would be the most single most important addition to the expected quality of your rocks that you could make assuming you don't sacrifice too much of the latter to get it. If you consider the life of a person in a poor region, the lack of sand is often the most salient feature that underpins their relation to the world and to others. At the extreme end, a homeless vagrant cannot escape preoccupation with sand and sees their life entirely directed by its lack. Those already at an enviable position on the Maslow hierarchy cannot see the value of the sand anymore because they've always had enough.